Heater fan on high - at random

[psq]

We're touring the south west in the 450SLC and its going well, except its acquired the habit of turning the heater fan on high regardless of any action by us. We might start in the morning and its running normally then a few ks down the road it suddenly bursts into life. A/C on or off, heater controls any where, makes no difference. This morning it went for a couple of hours normally, then abruptly turned up to full blast.
Any ideas? This laptop only has the W116 manual on it so I can't go looking for rampant relays, etc. as I don't know where to look.

[AMG]

Sounds like the fan speed controller (rheostat) under the blower motor cover in the engine bay.

Pretty easy to check. While you're there, make sure you vacuum out any debris, and clean everything as much as you can. It'd be advisable to pull the blower motor out and clean both the cowl and the blower motor separately. the electricals will more than likely need a good dose of CO cleaner.

then insofar as testing goes, you should check the earth for the fan speed controller, and the 3 separate resistances.

system-identification.pdf

description-hvac.pdf

blower-rr.pdf

74-blower-schematic.pdf

AutoCC-1980.pdf

[psq]

Looked all through the resistor cage (back at home now), all looks fine. In fact the only thing I can see as a failure point is that the resistor coils might corrode and break in which case the fan would not run on one or more speeds. The next time I'm in the shed I'll be checking that there's not any exposed wires in the multi-pin plug, but they looked fine at first glance.
To clarify the behaviour - I start the car, things are fine (most times) with the fan running at the speed selected on the switch. Anywhere from a few seconds to 20 minutes later the fan suddenly starts running at full (noisy) blast. Sometimes its on full blast from the start. The switch can be in any position, including off, and moving it makes no difference.
Any ideas?

[psq]

I'm getting no further with this, currently have the switch panel out, no obvious wires or vacuum out of place.
Is there any kind of defrost system that would not come on until the engine warmed a (very) little?

[John McMaster]

Hi, You may have to try resoldering the climate control board.
Good luck
JM

[psq]

Not a US car, no climate control per se.
Is there some way the system "decides" to turn on high speed fan for, say, demist? The fact that it waits a while suggests there's a vacuum and temperature balancing act going on somewhere.
My next idea is to unplug the leads that go into the connector to the resistor box at the bulkhead one at a time. Pity they're really hard to get to.

[psq]

Its ok, nobody panic, I found it.
There's 7 wires go into the plug at the bulkhead: 4 speeds, an earth, a link from the A/C, and a "pre-resistor". This last one seems to be a bimetallic strip that decides things are too hot and bypasses all of the resistors. It looks like its lost its marbles, because there's no heat in the resistors. Disconnect the wire, all is well.

[Michel]

Thank goodness you found it.
I was about to start panicking :x


Just kidding, been following that thread.

Glad you found the issue.

How wil you fix it? Or are you keeping it disconnected?

[TheMadRacoon]

Sounds like the fan speed controller (rheostat) under the blower motor cover in the engine bay.

Pretty easy to check. While you're there, make sure you vacuum out any debris, and clean everything as much as you can. It'd be advisable to pull the blower motor out and clean both the cowl and the blower motor separately. the electricals will more than likely need a good dose of CO cleaner.

then insofar as testing goes, you should check the earth for the fan speed controller, and the 3 separate resistances.
system-identification.pdfdescription-hvac.pdfblower-rr.pdf74-blower-schematic.pdfAutoCC-1980.pdf

I'm having a similar issue. Except my 350SLC doesn't have the original fan switch, rather a two position toggle, so I guess I'm only getting two fan speeds rather than four. Or was. It used to ne that they worked. Then they worked in only one position, not always the same one. Sometimes hitting a big bump would bring the fan to life (switch in either fan position but previously fan not running).

Recently neither position has brought on the fan, but if I've been cruising and coast down (eg to a service station) then as I pull up the fan comes on, on Hi. Even, iirc, if fan switch is on off position. It led me to suspect the switch.

Yesterday I switched on the AC and felt a slight breeze but thought it was just fresh air coming through the vents without fan assist. After checking the wiring diagram I suspect the AC brought on the lowest (ineffectual) fan speed to ensure evaporator doesn't freeze.

So I need to check the non-original fan switch and resistor box.....but just discovered my multimeter battery is flat.

If the resistances are out, can this resistor box be opened and resistors replaced? And the bimetallic switch....unless it's broken how do I test practically? What temperature does it trigger on?

Or is it ebay time for a replacement?

[Chai]

All except the highest fan speed has resistors to lower the fan motor voltage/current supply. These resistors heat up.

The bimetallic is working. It's purpose is to sense the resistors that limit the fan-speed are overheating. When too hot, the bimetallic switch bypasses that resistor and lets the fan run at full speed while the resistor cool down.

When the A/C runs, the lowest fan speed is enabled even when the switch is in the off position.

My guess is the carbon contacts for the motor are almost worn or motor bearings are seizing up, which is why the bumps cause the fan to occasionally work. Also explains the hi-speed running when toggle is switched off - likely the A/C was switched on at that time.

Read this: viewtopic.php?f=13&t=22244

[TheMadRacoon]

Thank you Chai, will read up on it.